1. Field of the Invention
The present invention relates to a voltage stabilization control method, and more particularly to an improvement in voltage stability of a power system in a voltage stabilization control method for the power system due to an adjustable speed machine.
2. Description of the Related Art
Hitherto, in order to improve voltage stability of the power system, there are used a control unit and a control system in which a transmission voltage is controlled to a constant value by an excitation control unit such as a PSVR (power system voltage regulator) or an HSVC (high side voltage control), or a reactive power is compensated by a phase modifier such as an SVG (statcom). In general, a relation between the transmission voltage and a terminal voltage of the power generator is represented by the following expression.
VH=Vgxe2x88x92Iqxc2x7Xtxe2x80x83xe2x80x83(1)
Where VH is a transmission voltage at a higher voltage side of a main transformer connected with a terminal of a synchronous machine, Vg is a terminal voltage of the synchronous machine, Iq is a reactive current, and Xt is a leakage reactance of the main transformer.
Since a normal voltage control controls the terminal voltage of the synchronous machine constantly, when the system voltage is lowered, the transmission voltage VH drops together with a drop of the system voltage as the reactive current Iq increases. On the contrary, according to the transmission voltage constant control method such as the PSVR or the HSVC, the transmission voltage can be maintained so as to compensate an amount as large as the dropped amount corresponding to the reactance of the transformer by increasing the terminal voltage of the synchronous machine. Also, in the phase modifier such as the SVG, the reactive current is compensated, thereby being capable of preventing the transmission voltage from dropping.
FIG. 8 is a simplified circuit diagram showing a known general transmission system. In the figure, reference numeral 100 denotes a synchronous machine of a sending end, 101 is a transmission line that connects a transmission end and a receiving end, 102 is a load of the receiving end, and 103 is a transmission line impedance. FIG. 9 shows a characteristic (P-V curve) of an active power to a voltage of the transmission system shown in FIG. 8. Referring to FIG. 9, reference symbol C11 is a PV characteristic of the transmission system, and C21 is a load characteristic. In a normal state, the power system operates at an equilibrium point of an intersection A of C11 and C12. When the transmission line impedance increases due to a transmission line fault or the like, the characteristic of the transmission system largely changes. For example, in the case where the transmission characteristic is reduced to C12 after the fault but the load characteristic cannot be extended over C22, the transmission characteristic and the load characteristic cannot intersect with each other, with the result that the operation equilibrium point is lost and a voltage drop or a voltage breakdown. In the case where a component of a non-linear load such as a constant power load or an inductor load is large, or in the case where a tap changer such as an LTC is limited, the possibility that such a situation may occur is high. In order to prevent this circumstance, the transmission characteristic, after the fault is extended up to C13, can intersect with the load characteristic. In other words, when the active power which is short circuited at the load is quickly supplied, the transmission system can not be saved from the voltage drop or the voltage breakdown. However, in the conventional voltage control and reactive power control, the active power cannot be supplied. Also, in the case of a synchronous machine, because control of the active power can be conducted only by a speed control system, the power cannot be controlled at a high speed.
As shown in FIG. 9, it is generally known that there is a voltage stability limit H in the transmission characteristic C11 that is in a steady operation state. However, when the operation equilibrium point becomes lower than H, the voltage fluctuates. The value of the stability limit H changes by using the HSVC, the PSVR or the phase modifier. There is a fear that the operation equilibrium point enters an unstable region due to acceleration of the generators connected to the transmission system after an accident has been removed, or a rapid request at the load side. In this case, it is necessary to suppress the power that rapidly increases in the transmission system, but control cannot be conducted so as to absorb an increasing power of the transmission system and maintain the voltage of the transmission system by only the conventional voltage control and reactive power compensation. Also, in case of a synchronous machine, because the control of the active power can be controlled by only the speed control system, the power cannot be controlled at a high speed.
As described above, in the case where the power is short circuited at the load side due to the transmission line fault or the load is rapidly changing, if the active power that is short circuited at the load can be rapidly supplied, the voltage drop of the transmission system or the voltage breakdown can be prevented. However, in conventional voltage control and reactive power control, because the active power cannot be supplied, when a fault or the like occurs, a voltage drop and a voltage collapse occurs in the transmission system.
Also, there is a fear that the operation equilibrium point enters an unstable region due to acceleration of the generators connected to the transmission system after a fault has been removed, or upon a rapid request for power at the load side. In this case, because it is impossible to absorb the increasing power of the transmission system at a high speed and maintain the voltage of the transmission system by conventional voltage control and reactive power compensation, there arises a voltage drop of the transmission system and a voltage collapse occurs.
The present invention has been made to solve the above-mentioned problems, and therefore an object of the present invention is to obtain a voltage stabilization control method and apparatus each of which is capable of stopping a voltage drop of the transmission system and preventing the voltage collapse by supplying an active power to a load at a high speed when the active power is short at the load, and absorbing the power when the power rapidly increases in the transmission system.
With the above object in view, the voltage stabilization control method of the present invention for controlling a voltage of a power system that is connected with an adjustable speed machine, comprises the steps of: detecting a change of a condition of the power system; outputting a control signal indicative of the amount of adjustment when the adjustment of the active power of the adjustable speed machine is required on the basis of the change of the condition of the power system; and receiving a steady operation command value with respect to the adjustable speed machine, generating a stabilization control signal resulting from adding the control signal to the steady operation command value and outputting the stabilization control signal to the adjustable speed machine.
Therefore, the active power of the adjustable speed machine connected to the power system is controlled at a high speed in correspondence with the active power change of the power system, thereby making it possible to suck or supply the power at the control target position and to stop the voltage drop of the transmission system, whereby the voltage collapse can be prevented.
Also, the change of the condition of the power system may comprise an active power change of the power system, a frequency change of the power system or a voltage change of the power system.
The present invention resides in a voltage stabilization control system for controlling a voltage of a power system that is connected with an adjustable speed machine.